LTC3107 1 3107f For more information www.linear.com/LTC3107 TYPICAL APPLICATION FEATURES DESCRIPTION Ultra-Low Voltage Energy Harvester and Primary Battery Life Extender The LTC ® 3107 is a highly integrated DC/DC converter de- signed to extend the life of a primary battery in low power wireless systems by harvesting and managing surplus en- ergy from extremely low input voltage sources such as TEGs (Thermoelectric Generators) and thermopiles. The step-up topology operates from input voltages as low as 20mV. Using a small step-up transformer, the LTC3107 provides a complete power management solution for typical wireless sensor applications that operate from a primary battery. The 2.2V LDO can be used to power an external microprocessor, while the main output voltage automatically adapts to match the voltage of the primary battery. The LTC3107 seamlessly transitions from battery power to harvested power whenever harvested energy is available, extending the life of the battery. The BAT_OFF indicator can be used to track battery usage. An optional storage capacitor accumulates excess harvested energy, further extending battery life. The LTC3107 is available in a small, thermally enhanced 10-lead (3mm × 3mm) DFN package. TEG Powered Thermal Harvester with Primary Cell Life Extender Percentage of Added Battery Life vs TEG Surface Temperature APPLICATIONS n Thermal Energy Harvesting Assisted Power Management System n V OUT Tracks the Primary Battery Voltage n 2.2V LDO Output n Reserve Energy Output, Clamped to 4.3V n Operates from Inputs as Low as 20mV n Battery In-Use Indicator (BAT_OFF) n I Q from Battery: n 80nA When Energy Harvesting n 6µA No Energy Harvesting n Standard, Compact Step-Up Transformer n Small, Thermally Enhanced 10-lead (3mm × 3mm) DFN package n Industrial Wireless Sensing n Remote Sensor and Radio power n HVAC n Automatic Metering n Building Automation, Security n Predictive Maintenance, Condition Monitoring L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and VLDO is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. 3107 TA01a C1 THERMOELECTRIC GENERATOR C2 SW GND 10μF 10μF V BAT V IN BAT_OFF CSTORE (OPTIONAL) LTC3107 VAUX VSTORE 3.6V T1 1nF 1:100 330pF + 2.2μF VLDO VLDO V OUT V OUT C OUT 3.6V + SURFACE TEMPERATURE (°C) T A = 23°C 25 BATTERY LIFE EXTENSION (%) 100000 35 30 50 45 40 3107 TA01 100 1000 10000 10 1 0.5Hz X MT RATE (227μW AVG) 1Hz X MT RATE (437μW AVG) 2Hz X MT RATE (679μW AVG) CUI CP20151 TEG 24×24×22mm HEATSINK NATURAL CONVECTION
Transcript
LTC3107
13107f
For more information www.linear.com/LTC3107
Typical applicaTion
FeaTures DescripTion
Ultra-Low Voltage Energy Harvester and
Primary Battery Life Extender
The LTC®3107 is a highly integrated DC/DC converter de-signed to extend the life of a primary battery in low power wireless systems by harvesting and managing surplus en-ergy from extremely low input voltage sources such as TEGs (Thermoelectric Generators) and thermopiles. The step-up topology operates from input voltages as low as 20mV.
Using a small step-up transformer, the LTC3107 provides a complete power management solution for typical wireless sensor applications that operate from a primary battery. The 2.2V LDO can be used to power an external microprocessor, while the main output voltage automatically adapts to match the voltage of the primary battery. The LTC3107 seamlessly transitions from battery power to harvested power whenever harvested energy is available, extending the life of the battery. The BAT_OFF indicator can be used to track battery usage. An optional storage capacitor accumulates excess harvested energy, further extending battery life.
The LTC3107 is available in a small, thermally enhanced 10-lead (3mm × 3mm) DFN package.
TEG Powered Thermal Harvester with Primary Cell Life ExtenderPercentage of Added Battery Life vs TEG Surface Temperature
applicaTions
n Thermal Energy Harvesting Assisted Power Management System
n VOUT Tracks the Primary Battery Voltage n 2.2V LDO Output n Reserve Energy Output, Clamped to 4.3Vn Operates from Inputs as Low as 20mVn Battery In-Use Indicator (BAT_OFF) n IQ from Battery: n 80nA When Energy Harvesting n 6µA No Energy Harvestingn Standard, Compact Step-Up Transformern Small, Thermally Enhanced 10-lead (3mm × 3mm)
DFN package
n Industrial Wireless Sensingn Remote Sensor and Radio powern HVACn Automatic Meteringn Building Automation, Securityn Predictive Maintenance, Condition Monitoring
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and VLDO is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners.
SW Voltage ................................................. –0.3V to 2VC1 Voltage (Note 5) ......................–0.3V to (VAUX+0.6V)C2 Voltage (Note 5) ......................................... –8V to 8VVAUX .....................................................15mA into VAUXVBAT, VSTORE ........................................... –0.3V to 4.5VVOUT, BAT_OFF .......................................... –0.3V to 4.5VVLDO ........................................................ –0.3V to 4.5VOperating Junction Temperature Range
(Note 2) ............................................. –40°C to 125°CStorage Temperature Range .................. –65°C to 150°C
(Note 1)
TOP VIEW
DD PACKAGE10-LEAD (3mm × 3mm) PLASTIC DFN
10
9
6
7
8
4
5
3
2
1 SW
C2
C1
BAT_OFF
GND
VAUX
VSTORE
VOUT
VBAT
VLDO
11GND
TJMAX = 125°C, θJA = 43°C/W, θJC = 5.5°C/W
EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB (Note 4)
orDer inForMaTionLEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime.Note 2: The LTC3107 is tested under pulsed load conditions such that TJ ≈ TA. The LTC3107E is guaranteed to meet specifications from 0°C to 85°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3107I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note that the maximum ambient temperature consistent with these specifications, is determined by specific operating conditions in conjunction with board layout, the rated thermal package thermal resistance and other environmental factors. The junction temperature (TJ)
PARAMETER CONDITIONS MIN TYP MAX UNITS
Minimum Harvester Start-Up Voltage Using 1:100 Transformer Turns Ratio 20 30 mV
Harvester No-Load Input Current Using 1:100 Transformer Turns Ratio, VIN = 20mV, All Outputs Charged and in Regulation
3 5 mA
Harvester Input Voltage Range Using 1:100 Transformer Turns Ratio l VSTARTUP 500 mV
VBAT Voltage Range l 2.0 4.0 V
VBAT Current Limit VOUT = 0V, VAUX = 0V (Battery Insertion) l 2 30 60 mA
The l denotes the specifications which apply over the specified operating junction temperature range, otherwise specifications are at TA = 25°C (Note 2). VAUX = 4V, VBAT = 3.6V unless otherwise noted.
is calculated from the ambient temperature (TA) and power dissipation (PD) according to the formula: TJ = TA + (PD • θJA°C/W), where θJA is the package thermal impedance.Note 3: Specification is guaranteed by design and not 100% tested in production.Note 4: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much higher than 43°C/W.Note 5: The Absolute Maximum Rating is a DC rating. Under certain conditions in the applications shown, the peak AC voltage on the C1 and C2 pins may exceed their Absolute Maximum Rating. This behavior is normal and acceptable because the current into the pin is limited by the impedance of the coupling capacitor.
pin FuncTionsVAUX (Pin 1): Output of the Internal Rectifier Circuit and VCC for the IC. Bypass VAUX with 10µF of capacitance. An active shunt regulator clamps VAUX to 4.3V (typical).
VSTORE (Pin 2): Output for the Optional Energy Storage Capacitor. A large capacitor may be connected from this pin to GND to store excess harvested energy, further ex-tending battery life in the event of an increase in load. It will be charged up to the maximum VAUX clamp voltage. If not used, this pin should be left open or tied to VAUX.
VOUT (Pin 3): Main Output of the Converter. When harvested energy is available, the voltage at this pin is regulated to 30mV below the voltage on the VBAT pin. If no (or insufficient) harvested energy is available to power the load, VOUT will be regulated to a voltage about 230mV below the voltage at VBAT. A large decoupling capacitor is usually required from VOUT to GND, to allow the output to ride-through short duration load transients without drawing current from the battery. A minimum capacitance value of 47µF is recommended for all applications. See the Applications Information section for details on sizing the capacitor.
VBAT (Pin 4): Primary Battery Input. This pin must be connected to a primary battery. This input will be used during start-up to bring VOUT into regulation (as well as VAUX and VLDO). After start-up, this input is used only as a reference voltage for VOUT, unless there is insufficient harvested power available, in which case it will power the
IC and the loads on VOUT and VLDO. A ceramic decoupling capacitor with a minimum value of 10µF is recommended from VBAT to GND.
VLDO (Pin 5): Output of the 2.2V LDO, which is powered from the higher of VOUT or VAUX. Connect a 2.2µF or larger ceramic capacitor from VLDO to GND. If not used, this pin should be tied to VAUX.
GND (Pin 6 and Exposed Pad Pin 11): Ground Pin for the IC. The exposed pad must be soldered to the PCB ground plane. It serves as a ground connection, and as a means of conducting heat away from the die.
BAT_OFF (Pin 7): Battery Off Output. This pin is an indicator of when the battery is in use. A logic low indicates that the battery is being used to assist in regulating VOUT. The pin will go high when VOUT is in regulation and the battery is not being used. It is not designed to source any current. It has an internal 1M pull-up resistor to VOUT.
C1 (Pin 8): Input to the Charge Pump and Rectifier Circuit. Connect a capacitor from this pin to the secondary winding of the step-up transformer.
C2 (Pin 9): Input to the N-Channel Gate Drive Circuit. Con-nect a capacitor from this pin to the secondary winding of the step-up transformer.
SW (Pin 10): Drain of the Internal N-Channel Switch. Con-nect this pin to the primary winding of the transformer.
operaTionThe LTC3107 is an ultra-low input voltage step-up DC/DC converter and power manager for extending the battery life of low power wireless sensors and other low power applications that utilize a primary battery. The LTC3107 intelligently manages harvested energy from sources such as TEGs (Thermo-Electric Generators) to service the output while minimizing battery drain, thereby maximizing battery life.
To simplify the adoption of energy harvesting by applica-tions presently powered by a primary battery, the LTC3107 is designed to use the voltage on the VBAT pin not only as an energy source to power the outputs in the absence of harvested input energy, but also as a voltage reference to regulate VOUT. In this way, the LTC3107 automatically adapts VOUT to track whatever battery voltage the applica-tion is already designed for, in the range of 2V to 4.0V.
The LTC3107 is suitable for extending the battery life in applications where the average power draw is very low, but where periodic pulses of higher load current may be required. This is typical of wireless sensor applications, where the quiescent power draw is extremely low a high percentage of the time, except during transmit bursts when circuitry is powered up to make measurements and transmit data.
The LTC3107 can also be used to trickle charge a standard capacitor or supercapacitor to store excess harvested energy when it is available. This further extends the life of the primary battery, by allowing the converter to ride-through periods of heavier load, or times when no harvested energy is coming in by drawing from this reservoir before switching over to the battery.
VBAT Input
The VBAT input should be connected to a primary battery with a voltage between 2V and 4.0V. Typical examples are 2 Alkaline cells, a single 3V Lithium coin cell, or a 3.6V Li-SOCl2 battery. These are representative batteries that would normally power the application without the benefit of energy harvesting.
The LTC3107 is designed to use the battery to start-up the IC and power VOUT and VLDO with or without any harvested energy available. If there is no harvester input, or insufficient harvester input to power the load, then VOUT will be provided by the battery through a current-limited switch internal to the LTC3107, and will be hysteretically regulated to a voltage 230mV below the battery voltage. The VLDO output will be fixed at 2.2V unless VBAT is below 2.2V, in which case it will track VBAT. When no harvested energy is available, the LTC3107 average quiescent current draw from the battery is typically 6µA.
If there is sufficient harvested energy available, then VOUT will be regulated to a voltage approximately 30mV below the VBAT voltage, and the battery will not be used to power VOUT. In this case, the battery current draw will drop to just 80nA typical.
Oscillator
The LTC3107 utilizes a MOSFET switch to form a resonant step-up oscillator using an external step-up transformer and a small coupling capacitor. This allows it to boost input voltages as low as 20mV up to values high enough to provide multiple regulated output voltages for powering other circuits. The frequency of oscillation is determined by the inductance of the transformer secondary winding, and is typically in the range of 10kHz-100kHz. For input voltages as low as 20mV, a primary-secondary turns ratio of about 1:100 is recommended. For higher input voltages, this ratio can be lower. See the Applications section for more information on selecting the transformer.
Charge Pump and Rectifier
The AC voltage produced on the secondary winding of the transformer is boosted and rectified using an external charge pump capacitor (from the secondary winding to pin C1) and the rectifiers internal to the LTC3107. The rectifier circuit feeds current into the VAUX pin, providing charge to the external VAUX capacitor. Once VAUX exceeds 2V, synchronous rectifiers in parallel with each of the diodes take over the job of rectifying the input voltage, improv-ing efficiency.
The active circuits within the LTC3107 are powered from VAUX, which should be bypassed with a capacitor of 10µF minimum.
The quiescent current draw on VAUX is typically just 6µA. If harvested energy is available, this current will come from the harvesting source. If there is no harvesting energy available, the VAUX supply current will come from VBAT.
A shunt regulator limits the maximum voltage on VAUX to 4.3V typical. It shunts to ground any excess harvested current into VAUX when there is no load on the converter or the input source is generating more power than is required by the load. If the optional storage capacitor is connected to VSTORE, then the excess current will be used to charge the storage capacitor, and current will not be shunted to ground until the storage capacitor is charged up to the 4.3V clamp level.
Voltage Reference
The LTC3107 includes a precision, micropower reference, for accurate regulated output voltages. This reference becomes active as soon as VAUX exceeds 1.9V.
Low Dropout Linear Regulator (LDO)
The LTC3107 includes a low current LDO to provide a regulated 2.2V output for powering low power processors. The LDO is powered by the higher of VAUX or VOUT, and requires a minimum of 2.2µF ceramic decoupling capacitor. Larger capacitor values can be used without limitation.
If the LDO is not being used, the VLDO pin should be tied to VAUX.
VOUT
The LTC3107 is designed to fit seamlessly into existing applications that run from a primary battery, while adding the benefit of energy harvesting to increase the life of the battery. The main output voltage on VOUT is designed to track the battery voltage on VBAT. If no harvested energy is available, or the energy is insufficient to maintain VOUT, then VOUT will be hysteretically regulated 230mV below VBAT by periodically connecting it to VBAT.
When enough harvested energy is available to power the load, VOUT will be hysteretically regulated to a voltage typically 30mV below VBAT, and the battery will not be used. In this condition, the current drain on the battery is only 80nA typical.
In a typical application, a bulk decoupling capacitor (usually a few hundred microfarads) is connected to VOUT to allow it to ride-through small, periodic load transients typical of a wireless sensor application. If the VOUT capacitor is sized appropriately (see the Applications Information section for more detail), and the average harvested input power exceeds the average load power, then battery energy will never be used.
BAT_OFF
The BAT_OFF output is a digital output with an internal pull-up to VOUT. BAT_OFF is an indicator of when the battery is being used to help maintain VOUT. If BAT_OFF is high, it indicates that VOUT (and VLDO) are being powered entirely by the harvested input power (including the VSTORE capacitor), and the battery is not being used. In this case, the battery current draw is only 80nA typical. When BAT_OFF goes low, it indicates that the battery is being used to help maintain VOUT and VLDO in regulation. This indicates that either there is no harvested energy available, or it is insufficient to power the load entirely. If the COUT capacitor is not sized properly, the BAT_OFF indicator may go low during a pulsed load event, to in-dicate that current is being drawn from the battery. See the Applications Information section of this data sheet for guidance on sizing the COUT capacitor.
VSTORE
The VSTORE output can be used to charge an optional storage capacitor, after VOUT has reached regulation. The VSTORE capacitor value can range from hundreds of micro-farads up to Farads. Once VOUT has reached regula-tion, the VSTORE output will be allowed to charge up to the maximum VAUX voltage if excess harvested energy is available. The storage capacitor on VSTORE can be used to power the system in the event that the input source is lost, or is unable to provide the current demanded by the loads on VOUT and VLDO, or simply to supplement the
VOUT capacitor to reduce VOUT ripple during load steps. The LTC3107 will automatically use energy from the VSTORE capacitor to maintain VOUT in regulation before drawing any current from the battery. Note that it may take a long time to charge a large VSTORE capacitor, depending on the harvested energy available and the loading on VOUT and VLDO.
If a storage capacitor is not being used, the VSTORE pin can be left open or tied to VAUX.
Short Circuit Protection
All outputs of the LTC3107 are current limited to protect against short circuits.
Output Voltage Sequencing
A diagram showing the typical output voltage profiles dur-ing start-up with an energy harvesting source is shown in Figure 1.
operaTionOperation with Battery Removed
Although the LTC3107 is designed to have a primary battery connected to VBAT, there may be times when the battery is removed for a short duration (such as for maintenance).
If there is sufficient harvested energy (or stored energy) available to maintain VOUT and VLDO, then the current draw on VBAT will be only 80nA, plus any leakage from the VBAT decoupling capacitor, which should be very small (typically less than 1µA). In this case, if the battery is removed, the capacitor on VBAT will hold-up the VBAT voltage, allowing VOUT to maintain regulation. As the VBAT voltage slowly decays due to leakage, VOUT will follow it. For example, if the VBAT decoupling capacitor is 20µF nominal, and the total leakage on VBAT is 0.1µA, then VBAT and VOUT will decay at a rate of 5mV per second.
If there is no harvested or stored energy available to power VOUT and VLDO, then these voltages will drop when the battery is removed. In this case, their rate of decay will be determined solely by the amount of capacitance on VOUT (since it is generally much larger than the VLDO capaci-tor) and the combined load current on VOUT and VLDO.
The LTC3107 is designed to gather energy from very low input voltage sources and use it to extend the life of a primary battery in applications such as wireless sensors. The LTC3107 is designed to accumulate and manage energy over long periods of time to enable short power bursts for acquiring and transmitting data. These bursts must occur at a low enough duty cycle such that the total output energy during the burst does not exceed the aver-age source power integrated over the accumulation time between bursts. In these instances, the battery will not be used at all, so the battery life may be extended up to the shelf life of the battery.
Harvesting Input Voltage Sources
The LTC3107 can operate from a number of low input voltage sources, such as thermoelectric generators, ther-mopiles and coil and magnet transducers. The minimum input voltage required for a given application will depend on the transformer turns ratio, the load power required, and the internal DC resistance (ESR) of the voltage source. Lower ESR sources (typically less than 10Ω) will allow the use of lower input voltages, and higher output power capability.
For a given transformer turns ratio, there is a maximum recommended input voltage to avoid excessively high secondary voltages and power dissipation in the shunt regulator. It is recommended that the maximum input voltage times the turns ratio be less than 50.
Note that a low ESR bulk decoupling capacitor may be required across the input source to prevent large voltage droop and ripple caused by the source’s ESR and the peak primary switching current (which can reach hundreds of milliamps). The time constant of the filter capacitor and the ESR of the voltage source should be much longer than the period of the resonant switching frequency.
A Peltier module (also known as a thermoelectric cooler) is made up of a number of series-connected P-N junctions, sandwiched between two parallel ceramic plates. Although Peltier modules are often used as coolers by applying a DC voltage to their inputs, they will also generate a DC output voltage, using the Seebeck effect, when the two plates are held at different temperatures. The polarity of the output voltage will depend on the polarity of the tem-perature differential between the plates. The magnitude of the output voltage is proportional to the magnitude of the temperature differential between the plates. When used in this manner, a Peltier module is referred to as a thermoelectric generator (TEG).
The low voltage capability of the LTC3107 design allows it to operate from a TEG with temperature differentials as low as 1°C to 2°C, making it ideal for harvesting energy in applications where a temperature difference exists between two surfaces or between a surface and the ambient temperature. The internal resistance (ACR) of most TEGs is in the range of 1Ω to 10Ω, allowing for reasonable power transfer. The curves in Figure 3 show the open-circuit output voltage and maximum power transfer for a typical TEG with an ACR of 2Ω over a 20°C range of temperature differential. It can be seen that an output power of a few hundred microwatts is easily achievable with a small temperature differential. This is often more than enough to satisfy the average power demand of a low power wireless sensor.
Note that the thermal resistance of most TEGs is typically quite low (2°K/W to 20°K/W). Therefore, it may be difficult to sustain a large temperature differential across the TEG. The temperature differential will depend on the amount of heat transfer available. In most applications, this will be determined by the size of the heat sink used on the TEG, and the amount of air flow. For optimal performance, the thermal resistance of the heat sink should be at least as low as the thermal resistance of the TEG being used.
applicaTions inForMaTionAlso note that most Peltier cells are limited to a maximum absolute temperature of around 125°C. For applications with a heat source above this temperature, a thermopile generator should be considered, as they are designed for much higher operating temperatures.
TEG Load Matching
The LTC3107 was designed to present a minimum input resistance (load) in the range of 2Ω to 10Ω, depending primarily on input voltage and transformer turns ratio (as shown in the Typical Performance Curves). For a given turns ratio, as the input voltage drops, the input resistance increases. This feature allows the LTC3107 to optimize power transfer from sources with a few Ohms of source resistance, such as a typical TEG. Note that a lower source resistance will always provide more output current capa-bility (all other things being equal) by providing a higher input voltage under load to the converter.
DELTA T (°C)ACROSS TEG
0
TEG
OPEN
CKT
VOL
TAGE
(V)
MAX P
OUT (mW
)
0.3
0.1
0.2
0
10
0.1
1
0.012010 155
3107 F03
25
MAX POUTVOC
TEGACR = 2Ω
Figure 3. Typical Performance of a Peltier Module Acting as a Power Generator
Table 1. Peltier Module Manufacturers MANUFACTURER Web Address
Marlow Industries www.marlow.com
Nextreme www.nextreme.com
Tellurex www.tellurex.com
Ferro Tec www.ferrotec.com
Z-Max www.z-max.jp/peltier_en/peltier
CUI www.cui.com
Laird Technologies www.lairdtech.com
Table 2. Peltier Module Distributors MANUFACTURER Web Address
Peltier modules are available in a wide range of sizes and power capabilities, from less than 10mm square to over 50mm square. They are typically 2mm to 5mm thick. A list of manufacturers that make Peltier modules appropriate for use with the LTC3107 is given in Table 1. Distributors that sell Peltier modules are shown in Table 2.
Thermopile Generator
Thermopile generators (also called powerpile generators) are made up of a number of series connected thermocouples, enclosed in a metal tube. They are commonly used in gas burner applications to generate a DC output of hundreds of millivolts when exposed to the high temperature of a flame. Typical examples are the Honeywell CQ200 and Q313. These devices have an internal series resistance of less than 3Ω, and can generate as much as 750mV open circuit at their highest rated temperature. For applications where the temperature rise is too high for a traditional Peltier thermoelectric device, a thermopile can be used as an energy source to power the LTC3107. Because of the higher output voltages possible with a thermopile generator, a much lower transformer turns ratio is recommended (typically 1:10 or 1:20, depending on the application).
COMPONENT SELECTION
Step-Up Transformer
The step-up transformer turns ratio will determine how low the input voltage can be for the converter to start. Using a 1:100 ratio can yield start-up voltages as low as 20mV. Other factors that affect performance are the DC resistance of the transformer windings and the inductance
applicaTions inForMaTionof the windings. Higher DC resistance will result in lower efficiency. The secondary winding inductance will deter-mine the resonant frequency of the oscillator, according to the formula:
FREQ= 12•π• L•C
Hz
Where L is the inductance of the transformer secondary winding and C is the load capacitance on the secondary winding. This is comprised of the input capacitance at pin C2, typically 30pF, in parallel with the transformer sec-ondary winding’s shunt capacitance. The recommended resonant frequency is in the range of 10kHz to 100kHz. Note that the loading also plays a role in the effective load capacitance, and will therefore have an effect on the fre-quency. See Table 3 for some recommended transformers.
Squegging
Certain types of oscillators, including transformer coupled oscillators such as the resonant oscillator of the LTC3107, can exhibit a phenomenon called squegging. This term refers to a condition that can occur which blocks or stops the oscillation for a period of time much longer than the period of oscillation, resulting in bursts of oscillation. An example of this is the Blocking Oscillator, which is designed to squegg to produce bursts of oscillation. Squegging is also encountered in RF oscillators and regenerative receivers.
In the case of the LTC3107, squegging can occur when a charge builds up on the C2 gate coupling capacitor, such that the DC bias point shifts and oscillation is extinguished for a certain period of time, until the charge on the capacitor bleeds off, allowing oscillation to resume. It is difficult to predict when and if squegging will occur in a given ap-plication. While squegging is not harmful, it reduces the average output current capability of the LTC3107.
Squegging can easily be avoided by the addition of a bleeder resistor in parallel with the coupling capacitor on the C2 pin. Resistor values in the range of 100k to 1MΩ are sufficient to eliminate squegging without having any negative impact on performance. For the 330pF capacitor used for C2 in most applications, a 499k bleeder resistor is recommended. See the Typical Applications schematics for an example.
Using External Charge Pump Rectifiers
The synchronous charge pump rectifiers in the LTC3107 (connected to the C1 pin) are low current and optimized for operation from very low input voltage sources, using typical transformer step-up ratios between 1:100 and 1:50, and typical C1 charge pump capacitor values less than 10nF.
Operation from higher input voltage sources (typically 250mV or greater, under load), allows the use of lower transformer step-up ratios (such as 1:20 and 1:10) and larger C1 capacitor values to provide higher output current capability from the LTC3107. However, due to the result-ing increase in rectifier currents and resonant oscillator frequency in these applications, the use of external charge pump rectifiers is recommended for optimal performance in these applications.
Table 3. Recommended TransformersVENDOR PART NUMBER
The charge pump capacitor that is connected from the transformer’s secondary winding to the C1 pin has an effect on converter input resistance and maximum output current capability. Generally a minimum value of 1nF is recommended when operating from very low input volt-ages using a transformer with a ratio of 1:100. Too large a capacitor value can compromise performance when operating at low input voltage or with high resistance sources. For higher input voltages and lower turns ratios, the value of the C1 capacitor can be increased for higher output current capability. Refer to the Typical Application schematic examples for the recommended value for a given turns ratio.
In applications where the step-up ratio is 1:20 or less, and the C1 capacitor is 10nF or greater, the C1 pin should be grounded and two external rectifiers (such as 1N4148 or 1N914 diodes) should be used. These are available as dual diodes in a single package, such as the BAS31. (Avoid the use of Schottky rectifiers, as their lower forward voltage drop increases the minimum start-up voltage.) See the Typical Applications schematics for an example.
VOUT Capacitor
For pulsed load applications where there is no energy available from the VSTORE capacitor, the VOUT capacitor should be sized to provide the necessary current when the load is pulsed on. The capacitor value required will be dictated by the combined load current on VOUT and VLDO (ILOAD), the duration of the load pulse (t), and the amount of voltage droop on the capacitor (ΔVVOUT). With the goal being to extend battery life as much as possible, the maximum capacitor droop should be less than the amount required to trip the BAT_OFF comparator (which will connect the battery to maintain VOUT). Therefore, the minimum recommended VOUT capacitor value in pulsed load applications is:
COUT(µF)= ILOAD(mA)• t(ms)
ΔVVOUT(V)
Where ΔVVOUT is typically 200mV. Note that even with a properly sized output capacitor, there must be more average harvested power available than the average load power requirement on VOUT to prevent using the battery. A minimum COUT capacitor value of 47µF is recommended for all applications, even if there is no pulsed load.
VSTORE Capacitor
The VSTORE capacitor, CSTORE, may be of very large value (thousands of microfarads or even Farads), to provide energy storage for times when the harvested input power may be lost or the load requirement is higher. Note that this capacitor can charge up to 4.48V max, so be sure that the holdup capacitor has a working voltage rating of at least 4.5V at the temperature that it will be used. The LTC3107 is designed to use energy from the VSTORE capacitor to maintain VOUT before using the battery.
applicaTions inForMaTion
Table 4. Recommended Storage CapacitorsMANUFACTURER SERIES
AVX BestCap Series TAJ, TPS Series Tantalum
Cap-xx G Series (Dual Cell) H Series (Dual Cell)
Cooper Bussmann KR Series KW Series PA, PB, PM, PH Series
CSTORE can be sized using the following equation (assum-ing no harvested energy or battery usage):
CSTORE(mF)= ILOAD(mA)• t(s)
(4.3– VBAT – 0.2)(V)
Where ILOAD is the average load on VOUT and VLDO combined. This assumes that the storage cap has had a chance to charge up to its typical clamp voltage of 4.3V.
To minimize losses and capacitor charge time, all capacitors used for VOUT and VSTORE should be chosen for low leakage, relative to the average load current in the application. See Table 4 for recommended storage capacitors. Note that leakage is generally worse at higher temperatures, so be sure to take into account the actual operating temperature the capacitor will see in the application.
PCB Layout Guidelines
Due to the rather low switching frequency of the resonant converter and the low power levels involved, PCB layout is not as critical as with many other DC/DC converters. There are however, a number of things to consider.
Figure 4. Example Component Placement for Two Layer PC Board
Due to the very low input voltage the circuit may operate from, the connections to VIN, the primary of the transformer and the SW and GND pins of the LTC3107 should be designed to minimize voltage drop from stray resistance, and able to carry currents as high as 500mA. Any small voltage drop in the primary winding conduction path will lower efficiency, as well as the minimum start-up voltage.
applicaTions inForMaTionAlso, due to the low charge currents available at the out-puts of the LTC3107, any sources of leakage current on the output voltage pins must be minimized. An example board layout is shown in Figure 4.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
package DescripTionPlease refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DD Package10-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1699 Rev C)
3.00 ±0.10(4 SIDES)
NOTE:1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT2. DRAWING NOT TO SCALE3. ALL DIMENSIONS ARE IN MILLIMETERS4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE5. EXPOSED PAD SHALL BE SOLDER PLATED6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
For more information www.linear.com/LTC3107 LINEAR TECHNOLOGY CORPORATION 2013
LT 1213 • PRINTED IN USALinear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LTC3107
relaTeD parTs
Typical applicaTionThermal Harvester for Higher VIN Applications, Using External Rectifiers
PART NUMBER DESCRIPTION COMMENTS
LT3009 3µA IQ, 20mA Linear Regulator VIN: 1.6V to 20V; VOUT(MIN): 0.6V to 19.5V, 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, 5V Fixed; IQ = 3µA; ISD < 1µA; 2mm × 2mm DFN-8 and SC70 Packages
LTC3103 LTC3104
15V, 300mA Synchronous Step-Down DC/DC Converter with Ultralow Quiescent Current
VIN = 2.2V to 15V, VOUT = 0.8V to 13.8V, IQ = 1.8µA, ISD < 1µA, 10mA LDO (LTC3104), DFN and MSOP Packages
LTC3105 400mA Step-up Converter with MPPC and 250mV Start-Up VIN: 0.2V to 5V; VOUT = 1.5V to 5.25V, IQ = 24µA, ISD < 1µA 3 × 3 DFN-10/MSOP-12
LTC3108, LTC3108-1
Ultralow Voltage Step-Up Converter and Power Manager VIN: 0.02V to 1V; VOUT = 2.5V, 3V, 3.7V, 4.5V Fixed; IQ = 6µA; 3mm × 4mm DFN-12 and SSOP-16 Packages
LTC3109 Auto-Polarity, UltraLow Voltage Step-Up Converter and Power Manager
VIN: .03V to 1V; VOUT = Fixed 2.35V to 5V, IQ = 7µA 4mm × 4mm QFN-20 and SSOP-20 Packages
